1. Field of the Invention
This invention relates to a matrix liquid-crystal display device. More particularly, it is concerned with a matrix liquid-crystal diaplay device which is effective for the display of images particularly in a TV set.
2. Description of the Prior Art
A cathode ray tube (CRT) has long been, and is still the most important device for image display. Research has, however, been energetically carried out on flat panel displays, including a flat CRT, in order to develop a substitute having a smaller volume than a customary CRT. A liquid-crystal display device is a kind of flat panel display (FPD), and has recently begun to attract special attention because of its lower consumption of driving power.
The liquid-crystal display of an image, particularly a TV picture, is usually accomplished by a device having the so-called XY matrix electrode construction adapted to display picture elements. According to this device, the intersection between the Y (e.g., column) electrode and the X (e.g., row) electrode is electrically selected to define a unit picture element, and the brightness of the dot is utilized to make a display.
Such a matrix display device is, however, likely to develop a cross-talk effect causing information at a selected dot to mix into an unselected dot. Various proposals have been made to solve this problem, and can be roughly classified into (1) the optimization of the waveform of driving voltage, and (2) the optimization of the properties of display elements. It is known that in the effective voltage driving of a liquid-crystal display device, the waveform of its driving voltage can be optimized by a voltage equalization method (1/{.sqroot.N+1} biasing) (Kawakami and Yoneda: Proceedings of the Japanese Society of Electronic Communication, Meeting for the Study of Image Engineering, IE-46, 1974). According to this driving method, a selected dot (V.sub.on) and an unselected dot (V.sub.off) have an effective voltage ratio: ##EQU1## in which N stands for the number of multiplexing, of which the reciprocal corresponds to a duty ratio. As is obvious from equation P1, the V.sub.on /V.sub.off ratio approaches 1 with an increase in the value of N, even if the proposed driving method is employed. This means that the effective voltage of a selected dot fails to be sufficiently higher than that of an unselected dot, and a liquid-crystal display device according to a voltage selection scheme, and not having so high voltage dependence of a contrast as a discharge tube is likely to involve various problems, such as a reduction in the contrast of a display, and a slower response. In order to overcome these problems, studies have been made for a two-frequency addressing method, a multiple matrix method, a method incorporating a switching element into picture elements, or the like, but none of them has matured into practical application as yet. There is, thus, no alternative but to optimize the properties of display elements.
A mode utilizing depolarization in a twisted-nematic layer (DTN) has recently been proposed as a mode of liquid-crystal display which is suitable for matrix driving. The DTN mode is a mode of display possessing the features of both of the conventional TN (twisted nematic) and DS (dynamic scattering) types, as described in a paper by Y. Ishii, one of the inventors of the present invention, et al. (Uchida, Ishii and Wada: Proceedings of the SID, Vol. 21/2, 1980). A liquid-crystal display device employing the DTN mode is described in J. V. Cartmell's U.S. Pat. No. 3,749,474 entitled "Oriented Light Scattering Display Device".
It is an object of this invention to provide a liquid-crystal composition exhibiting the properties suited for the display of images particularly in a TV set to thereby obtain a matrix liquid-crystal display device of the DTN type provided with display elements having the optimum properties. No liquid-crystal material providing any such specific property is generally known in the prior art. It is, however, well known that in a display device of the TN type, it is preferable to use a nematic liquid-crystal of positive dielectric anisotropy (M. Shadt and W. Helfrich: Appl. Phys. Lett., Vol. 18 No. 4 page 127, 1971), while the use of a nematic liquid-crystal of negative dielectric anisotropy containing a dopant is preferred in a display device of the DS type. Preferred liquid-crystal materials for a display device of the DS type are described in U.S. Pat. No. 4,155,872 by F. Funada, one of the inventors of this invention, et al.